The present invention relates to a process for producing 2,6-naphthalenedicarboxylic acid which is useful as the starting material for producing polyester resins by the oxidation of 2,6-diisopropylnaphthalene.
Hitherto, as the aromatic dicarboxylic acid used as the starting material in the production of polyester resins, terephthalic acid obtained by using p-xylene as the starting material has been used and terephthalic acid is produced industrially in a large amount.
However, in recent years, there is a number of demand for improving the quality such as the heat-resistance and mechanical properties of polyester resin, and for meeting the demand, the polyester resin obtained by using 2,6-naphthalenedicarboxylic acid as the starting material instead of terephthalic acid has attracted an attention. Accordingly, the development of an industrially profitable process for producing 2,6-naphthalenedicarboxylic acid (hereinafter referred to as 2,6-NDCA) has come to be demanded.
For finding a process for producing 2,6-NDCA, studies have been carried out hitherto in line with the production of terephthalic acid, and the oxidation of each member of 2,6-dialkylnaphthalenes, for instance, 2,6-dimethylnaphthalene, 2,6-diethylnaphthalene and the like in the presence of a known catalyst has been tried. Since 2,6-dimethylnaphthalene is easily oxidizable by a process similar to that for oxidizing p-xylene, it is convertible to 2,6-NDCA in a high yield by the oxidation process while using the known catalyst (refer to Examples 1 to 3 of Japanese Patent Publication No. 59-13495 (1984)). On the other hand, there is a practical problem that 2,6-dimethylnaphthalene itself is not easily available industrially. Namely, in the process for production of 2,6-dimethylnaphthalene comprising subjecting naphthalene to dimethylation to obtain the isomeric mixture of dimethylnaphthalenes and separating 2,6-dimethylnaphthalene from the isomeric mixture of dimethylnaphthalenes, the dimethylation is not easily effected and the separation is also difficult and accordingly, 2,6-dimethylnaphthalene cannot be said to be an industrially suitable starting material for obtaining 2,6-NDCA.
Concerning 2,6-diethylnaphthalene, the production thereof is easier than that of 2,6-dimethylnaphthalene (Japanese Patent Application Laid-Open No. 51-6953 (1976)), however, the diethylation of naphthalene and the separation of 2,6-isomer from the isomeric mixture of diethylnaphthalenes are not necessarily carried out easily in an industrial scale and accordingly, also 2,6-diethylnaphthalene cannot be said to be suitable as the starting material for 2,6-NDCA.
On the other hand, in consideration of the above-mentioned situations, 2,6-diisopropylnaphthalene which is easily separated from the isomeric mixture of diisopropylnaphthalenes easily available by the diisopropylation of naphthalene has been used as the starting material for 2,6-NDCA. However, as has been disclosed in Japanese Patent Application Laid-Open No. 51-6953, oxidation of 2,6-diisopropylnaphthalene (hereinafter referred to as 2,6-DIPN) to 2,6-NDCA has been regarded extremely difficult, and any report on the production of 2,6-NDCA from the isolated 2,6-DIPN as the starting material has not been published except for the following.
There is a disclosure concerning the oxidation of diisopropylnaphthalene slightly contained in a mixture of naphthalene, 1-isopropylnaphthalene, 2-isopropylnaphthalene diisopropylnaphthalene and other compounds, into naphthalenedicarboxylic acid (refer to Japanese Patent Publication No. 48-27318 (1973)). Although the object of the above-mentioned oxidation is to obtain naphthoic acids and/or naphthalenedicarboxylic acid, the yield of naphthalenedicarboxylic acid by the disclosure was only 2.0 parts by weight to 20 parts by weight of the mixture used as the starting material.
Concerning the oxidation of aromatic hydrocarbons substituted by two isopropyl groups, for instance, it has been reported that although the oxidation of 1,4-diisopropylbenzene to monocarboxylic acid (p-isopropylbenzoic acid) is promptly carried out, the oxidation of the thus formed monocarboxylic acid to dicarboxylic acid (terephthalic acid) is difficult (refer to J. P. Fortuin et al., Petroleum Refinery, 38(6), 189-193 (1959)).
In the case of oxidation of 2,6-DIPN, the oxidizing susceptivity of 2-isopropyl group is not the same as that of 6-isopropyl group, and the oxidation of 6-isopropyl-2-naphthoic acid (formed by oxidation of 2,6-DIPN) is accompanied by the various side reactions such as ring-opening. Accordingly, it is impossible to obtain 2,6-NDCA in a high yield from 2,6-DIPN by the oxidation under one reaction condition.
Namely, the object of the present invention is to provide a process for favorably producing 2,6-NDCA which is useful as the starting material for producing polyester resin, by using as the starting material 2,6-DIPN which is easily available in an industrial scale and specifying the reaction conditions in the oxidation of 2,6-DIPN.
As a result of the present inventors' studies concerning the reaction conditions of the oxidation of 2,6-DIPN to 2,6-NDCA, it has been found that (1) the optimum reaction conditions in the reaction for oxidizing 2,6-DIPN to 6-isopropyl-2naphthoic acid (2-isopropylnaphthoic acid) are quite different from the optimum reaction conditions in the reaction for oxidizing 6-isopropyl-2-naphthoic acid to 2,6-NDCA, and (2) the above-mentioned two reactions are carried out separately in the two steps while using the same catalyst under the same reaction conditions except for the amount of the catalyst, whereby 2,6-NDCA can be obtained from 2,6-DIPN in a high yield, in other words, in a high selectivity.
The present invention has been attained on the basis of the findings.